The manufacture of optical fiber typically uses one of two fundamental approaches. Both use rotating lathes, and accumulate pure glass material on a rotating preform by chemical vapor deposition or a modification thereof. The earliest technique deposited material on the outside of a rotating preform, and the preform usually started as a hollow tube with a slowly increasing diameter as the vapor deposited glass material accumulated on the outside of the solid tube. A significant advance in this technology occurred with the introduction of the so-called Modified Chemical Vapor Deposition (MCVD) process of MacChesney et al. in which the glass forming precursers are introduced into a rotating hollow tube and the glass material is deposited on the inside wall of the hollow tube. In this way exceptionally pure material can be produced at the critical core region. It also allows better control over the reaction environment.
The MCVD process has evolved to a highly sophisticated manufacturing technique and is widely used in commercial practice today. However, problems in the tube collapse phase of the MCVD process persist. One of those is the spontaneous formation of a bubble in the core of the preform near the end of the collapse cycle. The bubble essentially destroys the preform. From 5-10% of preforms typically suffer this fate.
The dopant used to increase the core index in most commercial MCVD preforms is germanium in the form of GeO.sub.2. Studies of the bubble formation phenomenon have established that the source of the bubbles is GeO vapor from the GeO.sub.2 in the doped core. GeO vapor is emitted during the entire collapse cycle but as collapse proceeds the internal tube pressure due to accumulated GeO vapor increases, sometimes to the point where it exceeds the surface tension of the softened silica tube. When that occurs the tube wall distorts in a bubble, and there is no satisfactory way of reversing the distortion. Such tubes are scrapped, resulting in a significant reduction in process yield and substantially increased cost. Collapse techniques which avoid bubble formation would represent an important advance in the commercial practice of MCVD.